476 GEOLOGY. 



lean ones has been determined. The basic igneous rocks are, on the 

 whole, perhaps somewhat richer in ores than the acidic class, but there 

 is no estabhshed law. Many acidic rocks bear more and richer ores than 

 many basic ones. The view here entertained is that both classes 

 are subject to regional enrichment through conditions connected with 

 their origin, as yet little known. 



Marine segregation and dispersion. — In the formation of the sedi- 

 mentary rocks from the primitive and igneous rocks there was notable 

 metaUic concentration in some cases, and even more notable depletion 

 in others. The ground-waters of the land, after their subterranean 

 circuits, carried into the water-basins various metallic substances in 

 solution. These were either precipitated early in the marine or lacus- 

 trine drift of the w^aters, or became diffused throughout the oceanic 

 body. In the main they appear to have been widely diffused, and either 

 to have remained long in solution, or to have been very sparsely deposited 

 through the marine or lacustrine sediments. As a rule, these sediments 

 seem to contain less of valuable ore material than igneous rocks, and 

 this is rational, for, as we shall see, the ground-water circulation of 

 the land tends to concentrate and hold back a part of the metallic 

 content of the land rocks so that only a residue reaches the sea. But 

 there are important exceptions to this general rule of sedimentary 

 leanness. 



The iron-ore beds of Chnton age ranging from New York to Alabama, 

 and appearing also in Wisconsin and Nova vScotia, form a stratum in 

 the midst of the ordinary sediments, and contain marine fossils. The 

 great ore beds of Lake Superior were originally of similar type, and so 

 are most other important iron deposits. It cannot be said, in most 

 cases, that these iron deposits are marine as distinguished from lacus- 

 trine or, lodgment deposits, but they are at least sedimentary. The 

 ferruginous material w^as originally disseminated widely through ante- 

 cedent land rocks, but was concentrated in the course of the sedimen- 

 tary processes. 



Limestone appears to have been sometimes enriched locally in lead 

 and zinc, and more rarely in copper, in the course of its sedimentation. 

 The lead and zinc regions of the Mississippi basin have been regarded 

 as dependent on such regional enrichment as a primary condition. 

 This locahzed enrichment has been attributed to solutions brought into 

 the sea from neighboring metal-bearing lands and precipitated by 



